The present invention relates to an apparatus for and the use of a closed-cycle chemical combustion process as the means for producing accurately controlled and repeatable high-power, low-frequency, underwater sound pulses.
In general, the most effective low-frequency ocean acoustic source techniques have been non-reciprocal methods deriving their excitation energy from chemical forms (solid explosives; gas combustion), pneumatic forms (air gun), hydraulic forms (water gun; hydrodynamic), mechanical vibrators (electrical; hydraulic drive), motor-driven hammer/acoustic diaphragms, electrical discharge forms (sparkers, boomers), and cavity implosion devices. The most thoroughly exploited of these technologies are pneumatic and hydraulic sources (for off-shore seismic exploration) and the electrical discharge forms (for off-shore sub-bottom profiling and shallow marine exploration). The electric arc discharge technique had recently been refined to provide higher energy density together with the important ability to generate efficient controlled-spectrum pulses in the frequency range of about 200-2,000 Hz at input energy levels of about 1,200 Joules per pulse. With appropriate further development, this technique offers the prospect for becoming a low-maintenance ocean acoustic pulse source capable of generating accurately timed acoustic pulse signals at lower frequencies and having an input energy level up to about 10,000 Joules per pulse. However, with a practical electrical-to-acoustical energy conversion efficiency of about 15 percent, other acoustic pulse source techniques having higher energy conversion efficiency become important alternatives, provided that they can meet the practical requirements of accurate pulse timing and accurate repetitive pulse wavelet generation.
Chemical energy sources offer the highest available energy density and, in general, because of their direct energy release in the water medium, are the most efficient in converting their chemical reaction potential energy to radiated acoustic energy. For example, large underwater explosions are estimated to transform more than 50 percent of their latent energy into the outgoing shock wave pulse; a conversion process aided by the nonlinear response effects of such a finite amplitude source mechanism. Nevertheless, such sources approach the ideal performance effectiveness since the radiation efficiency of a simple linear acoustic impulsive source is inherently limited to 50 percent. That is, half of the total source energy is stored in the incompressible near field (i.e. half of the total source energy goes into kinetic mass flow imparted to the immediately surrounding liquid medium). This stored energy may only contribute to the acoustic signal when the source motion reverses as in a bubble cavity collapse.
In contrast with the impractical nature of solid state or monopropellant liquid explosives for use as a source of high-power sonar system pulses, gaseous explosions are potentially more practical by virtue of their adjustable energy content, comparable energy conversion efficiency, pulse repeatability and timing accuracy, and safety. Several forms of flexible sleeve gas exploder devices have been used in marine seismic exploration with generally good success. These devices are typically fueled by an oxygen-propane mixture fed to the combustion chamber by one or more hoses from a remote supply source and ignited by one or more remotely-controlled spark plugs. Operation of these devices at near-surface water depths has been a major advantage in providing simple design and reliable performance. However, remote metering of the gas mixtures at depth can lead to improper gas mixture variations which result in unreliable ignition and significant differences in generated pulse energy. Use of hydrocarbon fuels also produces exhaust gases which are troublesome in sources that must operate at depth.
To circumvent this problem, several forms of oxygen-hydrogen flexible sleeve exploders have been devised for use in the ocean and in boreholes to depths of 4,000 feet and possibly deeper. The oxygen-hydrogen gas mixture used in some of these devices has been derived by electrolysis of in situ sea water whereas others used a self-contained supply of aqueous electrolyte. By this electrolysis method, the generated oxygen and hydrogen mixture is produced in approximately stoichiometric balance independent of the pressure and depth conditions, resulting in more accurate ignition, combustion energy uniformity, and acoustic pulse repeatability. Combustion energy reactions up to about 200 kJoules per pulse appear to be practical for typical sonar transducer depths and pulse repetition rates in the range of about one pulse per minute, or less frequent. Combustion reaction of a stoichiometric mixture of oxygen and hydrogen forms steam as the sole combustion product which, upon condensation, will return as water to the electrolyzer to be reused in a closed-cycle repetitive gas generation and combustion process. By selecting the aqueous electrolyte which produces the lowest practical amount of chemically irreversible by-products in the closed-cycle oxygen-hydrogen electrolysis process (i.e. the minimum excess non-combustible chemical dissociation components and the minimum corrosion contaminants from the electrolytic cell electrodes and electrolyte chamber), the oxygen-hydrogen combustion process can be made accurately repetitive and tolerant of long-term cyclic operation.
To date, none of these alternative source techniques have been found to be practical either because of cumbersome and inefficient hardware or because of their limited ability to generate the desired sound energy level at the low frequencies of interest with directional specificity.
The present invention, based upon a gas combustion source concept, provides the practical advantages of a high-energy density gas reaction, a simple and safe closed-cycle source of the necessary fuel and oxidizing gases, and associated means for achieving highly directional sound radiation based either upon the flame front velocity in the combustible gas mixture combined with the combustion chamber geometry and components or the use of separate gas combustion elements in a spatial array combined with prescribed ignition timing control.